According to sources, O'Brien, a six-year NBC Universal veteran, died of a brain aneurysm. She was 42.

"Our hearts go out to the family and friends of our beloved colleague Nora, who was respected and cherished by so many people in the entertainment community," NBC Universal said in a statement. "She'll truly be missed by all of us."

NBC, which is set to unveil its fall schedule to advertisers Monday, canceled all pilot screenings slated for Thursday. Production on "Parenthood" has been shut down for two days.

According to the Contra Costa Times, O'Brien, who was single, had been in the Bay Area for the past few weeks as part of a film crew shooting the pilot for "Parenthood," based on the 1989 movie. Several locations in southern Marin County have been used for filming.

O'Brien complained of dizziness and collapsed at about 9:20 p.m. She was pronounced dead shortly before 10 p.m. at Marin General Hospital, the newspaper said.

O'Brien came to NBC in January 2008, when she was named VP drama programing for Universal Media Studios, which merged with the network in December. At the studio, she worked on the upcoming NBC drama series "The Philanthropist," among other projects.

O'Brien joined NBC from sister cable network Sci Fi, where she served as VP original programing for four years. There, she developed episodic and longform programs, including the miniseries "The Lost Room," and worked as a programing executive on "Stargate" and "Stargate Atlantis."
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